Remove unused functions from FSX.

* subversion/libsvn_fs_x/low_level.h
  (svn_fs_x__parse_revision_trailer,
   svn_fs_x__unparse_revision_trailer): Drop as FSX rev / pack files don't
                                        have revision trailers.

* subversion/libsvn_fs_x/low_level.c
  (svn_fs_x__parse_revision_trailer,
   svn_fs_x__unparse_revision_trailer): Ditto.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635643 13f79535-47bb-0310-9956-ffa450edef68